In recent years the demand for rare gases, in particular Krypton and Xenon, is becoming very important. New applications and advances in electronics, medical, glass insulation etc. are greatly contributing to this high demand.
Krypton and Xenon are produced as the by-products of a cryogenic air separation plant. The basic recovery scheme is well known in the art. Since Kr and Xe are heavier than oxygen and will accumulate in liquid oxygen, the recovery technique usually calls for the refining of a liquid oxygen purge stream of the low pressure column of a double column cycle. The rare gases contained in the purge stream are further concentrated in a first concentrating column along with other heavy components in liquid oxygen such as hydrocarbons, CO2, nitrogen oxide etc. For safety considerations, the limit of this first concentrating operation corresponds to about 10% of the limit of flammability of hydrocarbons in oxygen. The first concentrated stream is then either treated in an on-site purification plant or transported to a central purification center where it is vaporized, heated and treated in a catalytic reactor at high temperature of about 500° C. to remove the hydrocarbons. This oxidation reaction forms CO2 and moisture. The mixture is then dried, its CO2 content is removed in an adsorber. The dried and CO2-free mixture is then cooled and distilled to yield the product which is usually a mixture of Kr and Xe. The product is then further refined to remove oxygen, argon and some other impurities such as CFC compounds, green house gases, remaining traces of hydrocarbons etc. and to yield pure Krypton and pure Xenon as final products.
Kr and Xe are present in very small concentration in atmospheric air (1.14 ppm Kr and 0.086 ppm Xe by volume). Therefore it is currently only economically viable to produce Kr—Xe in large oxygen plants, preferably above 1000 T/D and even larger.
If the purification portion of the process can be a standardized process to refine different types of first concentrated streams, either from an oxygen plant, nitrogen plant, low purity or high purity oxygen plant etc. then the same remark cannot be applied for the process involved to extract a stream containing Krypton and Xenon from the air separation columns. Indeed, because of the above-mentioned variety of air separation plants/processes, it is not possible to have one type of extraction process applicable for all types of air separation plants. For example, a plant producing gaseous oxygen product from the low pressure column would require a different type of rare gases extraction from a plant producing liquid oxygen product for pumping from the low pressure column.
Heavy industrial demand for oxygen for gasification, IGCC, GTL, oxyfuels have increased significantly the size of trains of oxygen plants. Because of the limitation of the size of distillation columns by transport regulations the technological trend in cryogenic process is shifting toward elevated air pressure plants wherein the feed air and the columns' pressure are at higher pressure than traditional oxygen plants. The triple column process is designed to address this type of application and there is a need to provide a technique for extracting rare gases from this type of process.
This triple column process is described in details in several patent such as U.S. Pat. No. 5,231,837, and U.S. Pat. No. 5,341,646.
The technique of recovering Krypton and Xenon from an oxygen plant have been covered extensively in several patents:
U.S. Pat. No. 6,776,004: this prior art taught the technique of recovering rare gases of a mixing column plant for oxygen production. The liquid purge of the low pressure column is treated in an enrichment column reboiled by the top gas of the mixing column to recover the rare gases.
PCT WO 2004/023054: air feeds to the high pressure column is separated into a nitrogen rich stream and 2 oxygen rich liquid streams: rare gases rich liquid and rare gases lean liquid. The rare gases-rich stream is treated in a column located above the crude argon column to yield a Krypton Xenon concentrate at the bottom.
U.S. Pat. No. 6,662,593: the rare gases in the feed air are confined in a rare gases rich liquid stream of the high pressure column and then its oxygen content is stripped in a side column to yield the rare gases concentrate stream. By extracting the rare gases prior to the final distillation in the low pressure column the oxygen product can be quite lean in rare gases and can then be pumped and vaporized to high pressure as final product without incurring losses of rare gases.
U.S. Pat. No. 6,612,129: Krypton and Xenon containing liquid from the high pressure column is partially evaporated in the top condenser of the side-arm argon column of the double column plant. The liquid purge and the vaporized streams of the condenser are then treated in an enrichment column to yield the Krypton Xenon concentrate at the bottom.
U.S. Pat. No. 6,220,054: a column is used to treat the bottom liquid of the crude argon column to yield final oxygen product which is depleted of Krypton and Xenon since the feed to the crude argon column is also depleted in Krypton and Xenon. A stream concentrated in Krypton and Xenon is extracted at the bottom of the low pressure column.
As can be seen, most of the prior art addressed the rare gases recovery for oxygen plant equipped with argon production for high purity oxygen and in some cases, mixing column. Those processes operate at relatively low pressure at about 1.5 to 2 bar in the low pressure column which would yield an air pressure of about 6 to 7.5 bar. Higher pressure than these values would deteriorate the distillation performance especially for the argon recovery. Elevated pressure plant in contrary produces low purity oxygen and operates at about 10 to 16 bar air pressure with the low pressure column operates at about 4 to 6 bar. In order to maintain a good oxygen recovery rate an intermediate column is used to generate more liquid nitrogen reflux from the top of the intermediate column.